The contactless transponder, as used for contactless chip cards or contactless tags, for example, frequently involves the use of “ASK modulation”. This is understood to be a radio-frequency signal which, with data present in digital form, alternates between a first and a second level and is thus modulated.
In the same way as the distinction is drawn for digital data between “yes” and “no” or “1” and “0” or “high” and “low”, a distinction is drawn between a high and a low amplitude. Two different types of modulation are currently in common use, namely ASK 100 and ASK 10. ASK 100 has a level difference of 100% and ASK 10 has a level difference of 10%.
One problem of ASK modulation can be seen in that a change in the distance between transmitter and receiver of the signal modulated in this manner also results in a change in the received amplitude at the receiver end while the amplitude of the transmitted signal remains the same. The same applies if the medium changes in the intermediate space between transmitter and receiver. To get around this problem, a “shunt controller” is used at the receiver end, as specified in EP 0398943 B1, and is used to control the input signal amplitude upstream of the demodulator.
A drawback of the arrangement described in this EP publication is that the presence of a signal pause, for example following a change from “high” to “low”, prompts such a controller to attempt to correct the level difference produced by the information which is to be transmitted. To get around this, the starting point already taken, at least within the company, is for identification of a signal change from “high” to “low” to be followed by a controller gain being frozen to a value before the signal change. As soon as another signal change back from “low” to “high” is identified, the freezing of the controller gain is lifted and the controller continues to operate without restriction.
This measure in turn has the drawback that, at the end of a “low”/“high” change, the controller gain is raised until the high amplitude has been reached. As soon as the high amplitude has been reached, the controller gain is lowered again. However, such a measure easily results in a control overshoot. Such an overshoot can in turn result in the demodulator identifying a state change, i.e. a signal change from “high” to “low”, which is in turn not actually present. Such identification of a signal change would ultimately result in incorrect data.